Removable platens for holding objects for observation under microscopes are well known to those of skill in the art. For instance, platens holding a plurality of glass slides which interfit with fixed microscope stages are available to hold a plurality of such slides. More specifically, platens for holding silicon wafers having a plurality of semiconductor chips imprinted thereon for viewing under scanning electron microscopes (SEM) are also known. For example, such platens have a three-point clip system to hold the wafer to the platen for insertion onto the SEM stage. A spring-loaded retaining means forces the wafer against a pair of fingers spaced apart from one another to securely hold the wafer in place.
While the existing wafer holder (such as the Model 1830T wafer holder manufactured by Amray Inc. of Bedford, Mass. functions adequately to hold the wafer securely in place, there are a number of disadvantages associated with currently available products. Firstly, the three-point positioning system provides relatively large variability in the positioning of the wafer on the wafer holder, or platen. Because the spring-loaded retaining means is quite small, and because the retaining means forces the wafer against a pair of spaced-apart fingers, there is very little surface area of contact between the wafer and the wafer holder. Secondly, the particular construction of the presently available wafer holders results in the operator's fingers being frequently in contact with the surface of the wafer, thereby possibly damaging or inducing contamination onto the semiconductor chips imprinted thereon. Lastly, calibration standards must be inserted into the microscope stage for equilibration of the microscope, and then removed when the wafer and wafer holder are inserted.
Various other mechanisms have been proposed for securely holding arcuate objects such as silicon wafers. For instance, U.S. Pat. No. 2,333,114, Meyer, discloses a vise for holding small articles, such as movements of time pieces. Similarly, U.S. Pat. No. 2,471,103, Franks et al, discloses a pair of stationary pins 14, 15 and a pair of movable pins 18. 19 which are adapted to securely hold a round work piece, such as watch movements. U.S. Pat. No. 4,711,438, Guarino, discloses a mask holding device capable of accepting masks of different size. The holder includes a clamp assembly having a spring-loaded lower clamping arm to hold the mask securely in place. A quick-acting vise is disclosed in U.S. Pat. No. 3,088,729 Marcus, wherein a pair of jaws are slidably retained upon a track for holding a work piece therebetween. While not deemed by Applicants to be especially relevant, the following patents disclose devices relating to this subject matter: U.S. Pat. No. 4,508,326, Andre, U.S. Pat. No. 4,252,303, Shimai, and Japanese Pat. No. 57-96527.
Current manufacturing protocol in the electronics industry requires that silicon wafers having from 200-600 semiconductor chips imprinted on each five-inch diameter wafer are inspected at from about 5-10 sites per wafer by a SEM. The SEM stage is controlled by computer to locate the identical site on each successive wafer platen. As many as 30 sites per wafer may be inspected for engineering quality control. If the location of the wafer retained by the platen is not virtually identical from wafer to wafer, the preset microscopic scans will not identify the same location, thereby vitiating the viability of the quality control procedures. Applicant has determined that with currently available wafer holders, the variability inherent in such holders results in as much as about +/-600 micron accuracy.
Calibration standards are required for high resolution in every SEM. Typically in the observation of silicon wafers for quality control, a dummy wafer and placed bearing the calibration standards is inserted in a wafer holder onto the SEM stage, and the SEM is then calibrated. Any variation in the insertion of the holder, the wafer, wafer thickness, etc., will induce calibration errors into the system.
Finally, the particular construction of the currently available wafer holder requires that a pair of metal or plastic tweezers be used to insert the wafer onto and remove from the wafer holder. Adequate access ports are not provided to enable the use of a vacuum wand--the desired method of handling such wafers. Therefore, a wafer holder or platen which reduces the location variability, reduces the likelihood of operator-induced contamination, and provides a convenient calibration method, as needed.